Grain Structure

 

Have you ever seen the surface of a broken piece of steel? Chances are you have, especially in this craft. You'll notice it doesn't break like glass, or splinter like wood. It's closer to tile or concrete; the broken surface will be a grainy texture with varying grain size (which, by the way, is important to the strength of the steel, so keep that in the back of your mind). 

Grains, is exactly what this is. Let me break it down for you (pun intended). 

The atomic structure of iron is like a lattice; imagine a cube, with one iron atom at every corner and one in the center. The corner of one cube also forms the corner of another cube, so it's a continuing lattice, with iron atoms continuing at right angles to each other in three dimensions. A group of iron atoms, all linked together in this formation, is a single grain. 

The only reason a large bar of steel is not all a single "grain", is because one groups of atoms on one part of the bar will form their formations at a slightly different angle to one at the other. The place where grain A and grain B intersect then, is a weak point. And if enough stress is put on the steel to break it, it will be along these intersections. Thus, when you look at the broken surface of steel, you can see these boundary lines. 

Note: another way to see these grains, and perhaps the one most used for study, is to cut and polish the surface of the steel, then etch it with acid. Because some grains are orientated one way, and another a different way, the surface of one grain will etch away faster than another, making the surface of different levels depending on the grain. This will then show up under a microscope.  

The smaller yet more numerous the grains, the stronger the steel will be. The process of normalizing, which you are likely familiar with, changes the iron from "Ferrite" (it's natural room temperature formation state), to "Austenite" by heating to around 1600 F or so, then left to air cool. Every time it reaches the transitioning temperature, new grains from along the borders of the old. Heating and letting air cool results in the formation of multiple new grains, which are small yet numerous. I'll get into the details of the why's and how's of normalizing in a future post. 

For now, this should give you a full understanding of what grains are, and the true atomic structure of steel. Having a deep understanding of the scientific portion of bladesmithing gives you an intuitive knowledge of the steel, which helps with troubleshooting, estimating time and deforming rates, and exactly what you need to do to get the results you want.